Season 5 will begin airing in 2018. It will be special in that 2018 will celebrate 5 years of Thomas1Edward1Henry3's channel. It will also likely be the last season of The Engines of Sodor.
It is possible that T1E2H3 will upload the episodes in clusters, rather than one after the other. This allow more time for production.
After Season 5 Edit
The fifth season will conclude at the end of 2018. It is possible that T1E2H3 will conclude the Engines of Sodor here so he can focus on his film career. He will still be a member of the fandom, and may even turn to modeling characters as a hobby. However, in the future when he gets his own house, he may begin a new Thomas series, using something other than Trainz.